Set last known app theme background color until theme is loaded

index.html

@@ -28,6 +28,19 @@
 <noscript>You need to enable JavaScript to run this app.</noscript>
 <div id="root"></div>
 <script type="module" src="./src/index.tsx"></script>
+<script>
+    const backgroundColor = (() => {
+        try {
+            return JSON.parse(window.localStorage.getItem("theme_background"))
+        } catch (e) {
+            return null
+        }
+    })();
+
+    if (backgroundColor) {
+        document.documentElement.style.backgroundColor = backgroundColor;
+    }
+</script>
 <!--
   This HTML file is a template.
   If you open it directly in the browser, you will see an empty page.